Spatial light modulators (SLM's) have found numerous applications in the areas of optical information processing, projection displays, video and graphics monitors, televisions, and electrostatic printing. SLM's have also found uses as optical switches, optical shutters, light valves, pixel steerers and so forth. SLM's are devices that modulate incident light in a spatial pattern to form a light image corresponding to an electrical or optical input. The incident light may be modulated in its phase, intensity, polarization, and/or direction. The light modulation may be achieved by a variety of materials exhibiting various electro-optic or magneto-optic effects, and by materials that modulate light by surface deformation.
The present invention relates to SLM's of the foregoing type which may be used in a variety of devices, including light switches, light valves, pixel steerers and optical shutters.
A recent innovation of Texas Instrument Incorporated of Dallas, Tex. is the digital micromirror device or deformable mirror device, collectively known as the DMD. The DMD is a spatial light modulator comprising a monolithic single-chip integrated circuit, typically having a high density array of 17 micron square deflectable micromirrors but may have other dimensions. These mirrors are fabricated over address circuitry including an array of memory cells and address electrodes. The mirrors may be bistable and be operated in the digital mode, the mirror being stable in one of two deflected positions. A source of light directed upon the mirror is reflected in one of two directions by the mirror. When used in the digital mode, incident light from the array of mirrors can be modulated and directed to a projector lens and then focused on a display screen or a photoreceptor drum to form a light image. When the mirror is in one position, know as the "on" position, light is directed into the projector lens. In the other position, know as the "off" mirror position, light is directed to a light absorber. The DMD may also be monostable and operated in the analog mode, and finds use as a light switch, pixel steerer, optical shutter, scanner, and the like.
For a more detailed discussion of the DMD device and systems incorporating the device, cross reference is made to U.S. Pat. No. 5,061,049 to Hornbeck, entitled "Spatial Light Modulator and Method", U.S. Pat. No. 5,079,544 to DeMond, et al, entitled "Standard Independent Digitized Video System"; and U.S. Pat. No. 5,105,369 to Nelson, entitled "Printing System Exposure Module Alignment Method and Apparatus of Manufacture", each patent being assigned to the same assignee of the present invention, and the teachings of each are incorporated herein by reference. Gray scale of the pixels forming the image may be achieved by pulse width modulation techniques of the mirrors, such as that described in U.S. Pat. No. 5,278,652, entitled "DMD Architecture and Timing for Use in a Pulse-Width Modulated Display System", assigned to the same assignee of the present invention, and the teachings of each are incorporated herein by reference.
Commonly assigned U.S. Pat. No. 4,662,746 to Hornbeck entitled "Spatial Light Modulator and Method", 4,710,732 to Hornbeck entitled "Spatial Light Modulator and Method", U.S. Pat. No. 4,956,619 to Hornbeck entitled "Spatial Light Modulator", and U.S. Pat. No. 5,172,262 to Hornbeck entitled "Spatial Light Modulator and Method" disclose various structures and methods of producing micro mechanical devices, specifically, monostable DMD SLM's suited for use in the analog mode, the teachings of each incorporated herein by reference.
Commonly assigned U.S. Pat. Nos. 5,096,279 to Hornbeck et al. entitled "Spatial Light Modulator and Method", U.S. Pat. No. 5,142,405 to Hornbeck entitled "Bistable DMD Addressing Circuit and Method", and U.S. Pat No. 5,212,582 to Nelson entitled "Electrostatically Controlled Pixel Steering Device and Method" disclose various structures and methods for producing the same that are bistable and suited for use in the digital mode, the teachings of each incorporated herein by reference.
Referring to FIGS. 1A-1H, these embodiments being disclosed in a commonly assigned U.S. Pat. No. 5,172,262, there is shown a monostable DMD spatial light modulator that can be operated in the analog mode. One pixel, generally denoted at 20, is basically a flap covering a shallow well and includes a silicon substrate 22, a spacer 24, a hinge layer 26, a pixel layer 28, a flap 30 formed in layers 26-28, and plasma etch access holds 32 in flap 30. The portion 34 of hinge layer 26 that in not covered by pixel layer 28 forms a hinge attaching flap 30 to the portion of layers 26-28 supported by spacer 24. Pixel 20 is fabricated using a robust semiconductor process upon silicon substrate 22. Spacer 24 may be an insulating positive photoresist or other polymer, hinge layer 26 and pixel layer 28 are both an aluminum, titanium and silicon alloy (Ti: Si: AL), although these layers could also comprise of titanium tungsten, or other suitable materials. The hinge layer 34 may be about 800 Angstroms thick, wherein the pixel 30 is much thicker to avoid cupping and warping, and may have a thickness of about 3,000 Angstroms.
Pixel 20 is operably deflected by applying a voltage between mirror 30 and an underlying address electrode 36 defined on substrate 22. Flap 30 and the exposed surface of electrode 36 form the two plates of an air gap capacitor, and the opposite charges induced on the two plates by the applied voltage exert electrostatic force attracting flap 30 to substrate 22. This attractive force causes flap 30 to bend at hinge 34 and be deflected toward substrate 22. Depending on the opposing surface area of the electrodes, the spacing therebetween, the differential voltage applied, and the compliance of hinge 34, the degree of deflection of mirror 30 will vary. The deflection of mirror 30 is a function of the differential voltage, as graphically illustrated in FIG. 1C. As shown, the greater the differential voltage, that is, the greater the voltage applied to mirror 30, the greater the degree of deflection.
As also illustrated in FIG. 1 C, this deflection is nonlinear, and is not proportional to the voltage applied. A linear response, which may be the ideal response, is shown by the dotted line generally depicted at 38. The nonlinear relationship is due to many reasons. First, the electrostatic force is a function of the inverse of the square of the distance separating the mirror 30 and address electrode 36. Secondly, the geometry and composition of the hinge affects the compliance of hinge 34. The thickness of mirror 30 prevents significant warping, but the thinness of hinge 34 allows for large compliance. The deflection of flap 30 is a highly nonlinear function of the applied voltage because the restoring force generated by the bending of hinge 34 is approximately a linear function of the deflection, but the electrostatic force of attraction increases as the distance between the closest corner of flap 30 and electrode 36 decreases. Recall that the capacitance increases as the distance decreases so the induced charges both increase in quantity and get closer together. As shown in FIG. 1C, the voltage at which mirror 30 becomes unstable and bends all the way to touch and short with electrode 36 is called the collapse voltage. The analog operating region is that region between zero deflection and the collapsed situation.
FIG. 1D-1H illustrate equivalent alternative embodiments of the cantilever or leaf-type mirror 30 shown in FIG. 1.
When operating a spatial light modulator, such as of the type just discussed and referenced in FIG. 1A-1H, it may be desired to operated the deflectable member in the analog region, whereby the angle of deflection of mirror 30 is linearly proportional to the voltage applied. To operate the device as a light beam steerer, scanner, or light switch, it is desirable to precisely control the degree of deflection as so to precisely steer incident light to a receiver, such as a sensor. Therefore, in prior art designs, such as that shown in FIGS. 1A-1H, it is imperative that a repeatable process be followed. In the practical world, however, process tolerances allow for some deviation form device to device. Thus, for a given voltage being applied to address electrode 36, the deflection of mirror 30 from device to device will vary slightly. Consequently, characterization of the device prior to implementation is necessary when the device is used in the analog mode.
It is desired to provide a spatial light modulator with a deflectable pixel well suited to be used in the analog mode. The deflection of the pixel as a function of the address voltage should be easily characterized, consistent from device to device even with process tolerances, and approximately linear throughout a large range of deflection.